1. Field of the Invention
The present invention relates to a process according to the preamble of claim 1, for preparing a heterogeneous catalyst comprising a support and at least one catalytically active species bound to its surface.
According to such a process, the surface of the support is optionally first pretreated. The catalyst reagent containing the catalytically active species or its precursor is vaporized and the vapour is conducted to a reaction chamber where it is contacted with the support. The catalyst reagent not bound to the support is then withdrawn in gaseous form from the reaction chamber. If necessary, the species bound to the support is posttreated in order to convert it into a catalytically active form.
2. Description of the Related Art
As far as the prior art is concerned, reference is made to the following publications:
1. GB Patent Specification No. 1,105,564 (1968)
2. U.S. Pat. Specification No. 4,262,102 (1981)
3. U.S. Pat. Specification No. 4,362,654 (1982)
4. U.S. Pat. Specification No. 4,380,616 (1983)
5. Kase, A., Asakura, K., Egawa, C. and Iwasawa, Y., New Pd/Ultra-Thin Amorphous-Oxide Layer/ZSM-5 Catalysts for Selective Formation of Propane from CO/H2, Chem. Lett. 6 (1986) 855-858
6. Asakura, K. and Iwasawa, Y., The Surface Structure and Catalytic Properties of One Atomic Layer Amorphous Niobium Oxide Attached on Silicon Dioxide, Chem. Lett. 6 (1986) 859-862
7. Asakura, K. and Iwasawa, Y., New Reversible Enhencement/Depression Phenomenon on Catalysis of Platinum Supported on One Atomic Layer Niobium Oxide for Ethane Hydrogenation, Chem. Lett. 4 (1988), 633-636
8. Asakura, K, Aoki., M. and Iwasawa, Y, Selective Isopentane Formation from CH3OH on a New One Atomic Layer ZrO2/ZSM-5 Hybrid Catalyst, Catalysis Lett. 1(1988), 395-404
9. McDaniel, M. P., The State of Cr(VI) on the Phillips Polymersation Catalyst II: The Reaction between Silica and CrO2Cl2, Journal of Catalysis 76 (1982) 17-28
10. U.S. Pat. Specification No. 4,439,543 (1984)
Traditionally, heterogeneous catalysts have been prepared by depositing catalytically active compounds from the liquid phase onto the surface of the support by means of impregnation, precipitation or ion exchange. The starting materials used here comprise chemical compounds, often salts, which are soluble in known solvents. The solvents most frequently employed are water and different alcohols.
One drawback of the conventional technology is the large number of stages involved in catalyst preparation. The preparation of catalysts is recognised as an extremely delicate procedure requiring very accurate control of each requisite stage of the process.
Another drawback of prior art methods is associated with the need for solvents. The solvents by themselves often react with the support causing changes to the surface structure. This is particularly the case with the use of zeolites as the support material. The acidity of the surface has a decisive effect on the activity of the catalyst. The acidity is influenced both by the type of the acid sites, for example, the Brxc3x6nstedt and Lewis type, as well as by the number of sites. The acid sites can be influenced by, e.g., different heat treatments. When zeolites are treated with solvents, especially water, after a heat treatment, a definite change in the distribution of the acid sites is discernible. At least some of the acid sites then assume reversibly different forms. Thus, it is clear that the degree of acidity cannot be controlled during impregnation or ion exchange.
In addition to the above-mentioned drawbacks, the solvents used are often contaminated with impurities that can adversely affect the activity of the catalyst.
In order to eliminate the cited drawbacks of the liquid phase preparation processes, a number of different gas phase processes have been developed.
Reference [1] outlines a process that involves heating rhenium heptoxide to a temperature in the range from 150xc2x0 to 700xc2x0 C. and allowing the vapor to condense on the surface of an aluminum oxide support which is maintained at a temperature below 50xc2x0 C. Alternatively, the reaction is carried out at a temperature between 500xc2x0 and 600xc2x0 C., the Re2O7 partially decomposing to rhenium metal and forming a metal deposition on the alumina. The citation includes an example disclosing the preparation of a catalyst containing 14% Re2O7.
In the following three references [2, 3 and 4], processes for preparing silica-supported chromium catalysts have been described. The U.S. Pat. Specification No. 4,262,102 presents a method that involves vaporizing elemental chromium by heating it to a temperature of from 1400xc2x0 to 1700xc2x0 C. in a high vacuum metal evaporator, the chromium sublimating from the vapour phase on a chilled support in the form of small particles [2]. The procedures described in U.S. Pat. Specifications Nos. 4,362,654 and 4,380,616 comprise placing the silica support and a piece of chromium metal in a round-bottomed flask, evacuating the flask, and stirring the silica in the flask with a magnetic stirrer. The chromium is vaporized by heating with heat resistances [2 and 4]. In this case also, the chromium adheres to the support surface in the form of small particles.
In processes of the above type, the dispersion of the active metal may be heterogenic and there is not yet a proper understanding of how the carbene complex is formed on the catalyst.
In the Department of Chemistry at Tokyo University, researchers have prepared different catalysts having extremely thin layers, known as xe2x80x9catomic layersxe2x80x9d, of metal oxide bound to the surface of the supports [5-8]. Ideally, the catalysts comprises 1 to 3 of these atomic layers. Reference [5] discloses the preparation of catalysts having ultra-thin La2O3, TiO2, SiO2, and Nb2O5 layers on the outer surface of a zeolite (ZSM-5). Reference [6] describes the corresponding catalysts having a SiO2 support. According to reference [5], catalysts containing SiO2 and TiO2 are prepared by contacting methyltriethoxysilane and titanium isopropoxide vapors with the hydroxyls of ZSM-5 surfaces at 473 K (200xc2x0 C.) in a vacuum. Similarly, the ZrO2/ZSM-5 hybrid catalyst was prepared by contacting vaporized Zr tetraoxide having a vapour pressure of 133 Pa at 473 K with ZSM-5 at the same temperature. By repeating the binding reaction about 3 times, in both cases a single atom oxide layer covering the whole surface of the support was obtained.
According to the authors of the cited articles, the catalysts thus prepared have unique properties. Inwparticular, the catalysts exhibit good selectivity. Thus, the catalysts prepared according to reference [5] are used for selective propane preparation from CO and H2. The catalyst described in reference [6] is used during ethanol dehydrogenation, this catalyst being more active and selective than a catalyst prepared by impregnation or Nb2O5. The catalyst cited in reference [8] activates the formation of isopentane from methanol.
The common feature of the methods cited in references [5] to [8] is that first, a thin oxide layer is prepared on the surface of the support. The starting metal compound is chosen such that it does not fit into the cavities of the zeolite [5, 8].
McDaniel has studied the state of chromium(VI) on a Phillips polymerisation catalyst [9] and he has also, together with Stricklen, patented a process for preparing a CO-reduced chromyl halide silica-supported catalyst [10]. The starting compounds chosen include CrO2Cl2, CrO2F2 and CrO2FCl. Before depositing the starting compound on the support, the surface of the support was heated in an oxidising atmosphere, such as air, at a temperature within the range of 400 to 1000xc2x0 C. in order to remove the hydroxyl groups on the support. After the oxidising treatment, the oxygen was purged by nitrogen or argon gas flushing. At normal pressure and at a temperature ranging from 100 to 400xc2x0 C., a known amount of the reagent, in the example 0.5 to 4.0 ml of chromyl chloride, was then injected into a stream of nitrogen gas conducted through the support, the reagent vapours reacting with the hydroxyl groups of the support. Unreacted reagent vapor was withdrawn from the reactor.
The surfaces of the support particles used in heterogeneous catalysts are structurally inhomogeneous. As far as zeolites are concerned, the crystalline structure of the material is also complex containing, e.g., pore openings having diameters ranging from 0.3 to 0.7 nm. As a consequence of the above characteristics, the surfaces of the support materials are chemically inhomogeneous containing numerous binding sites of different valencies for new atoms or molecules contacted with the surface of the support. It is difficult to control the binding of the metals or metal compounds used as reagents when following any of the conventional practices described above in references [5] to [10].
The conventional processes also present difficulties in achieving homogeneous dispersion of the metals or the metal compounds on the support surfaces.
The present invention aims at eliminating the drawbacks of the prior art and at providing an entirely novel technical solution for gas phase preparation of heterogeneous catalysts.
The invention is based on the concept of bringing the catalyst reagent in-vapour state to the reaction chamber in an amount that at least corresponds to the amount of binding sites on the support surface. It is preferred to use an excess of reagent in relation to the available surface binding sites. The temperature of the support is kept higher than the condensation temperature of the vapor and sufficiently high for the active species or its precursor to be chemisorbed on the surface of the support. In other words, the process aims at providing the thermal activation energy needed for the formation of bonds between the active species or its precursor and the surface of the support.
In particular, the process according to the invention is mainly characterised by the steps of optionally pretreating a surface of a support material, vaporizing a reagent containing a catalytically active species, or a reagent containing a precursor of a catalytically active species, to form a vapor, introducing this vapor into a reaction chamber containing the support material held at a higher temperature than the vaporization temperature of the catalyst reagent, and binding the catalytically active species to the support material under temperature, pressure, and reaction time conditions sufficient to saturate the available support binding sites, removing any unreacted catalytic reagent vapor and optionally posttreating the catalyst.
The apparatus for preparing the heterogeneous catalyst according to the invention is characterised by a reactor body, a gas tight reaction chamber fitted within the reactor body and adapted for holding the support material, a plurality of heaters placed around the reactor body whereby the desired temperatures in the reactor body and in the reaction chamber are obtained, inert gas feed piping and inert gas outlet piping connected to the reaction chamber, wherein said inert gas feed piping is at least partially located within the reactor body, whereby a protective gas atmosphere inside the reaction chamber is obtained, and whereby the material flowing in the inert gas feed piping can be heated to the desired temperature by the heaters, gaseous reagent sources connected to the reaction chamber by the inert gas feed piping whereby the catalytically active compound is fed to the reaction chamber, and means for withdrawing unreacted gaseous reagent surplus.